Machines such as, for example, motor graders, wheel loaders, dozers, backhoes, dump trucks, and other heavy equipment are used to perform many tasks. To effectively perform these tasks, the machines require an engine that provides significant torque through a transmission to one or more ground engaging devices, such as the grader portion of a road grader. Often, these machines utilize continuously variable transmissions (CVT) for increased engine efficiency.
A CVT is an automatic type of transmission that provides an infinite number of output ratios within its ratio range. For example, a hydraulic CVT includes a pump and a fluid motor that receives pressurized fluid from the pump. Depending on a discharge flow rate and pressure of the pump, the motor speed and output torque at the ground engaging device may be varied.
CVT transmissions are becoming increasingly popular. The CVT allows the engine to maintain a narrow band of output velocities while varying the speed of the output drive shaft of the transmission. This allows the engine to run at more efficient revolutions per minute which can lead to better fuel economy than other transmissions. The CVT can also be used to maximize the performance of a vehicle by allowing the engine to turn at the RPM at which it produces peak power. In addition, the infinite gear ratios of a CVT allow for a smoother angular acceleration of the drive shaft than is possible with a traditional transmission which has a set number of discrete gear ratios. A disadvantage of the CVT is that it is traditionally configured without any decoupling devices, i.e., the transmission's input is always coupled to its output. Thus, the transmission continually transmits power from the engine to the vehicle, even during times when the engine is not capable of providing rated power, such as during startup of the engine.
In existing systems of vehicles such as motor graders, with direct drive transmissions, a cold start bypass system has been used to relieve oil system pressure on the drive train during a cold start of an engine. This cold start bypass results in lower transmission parasitic loss during a cold start and allows quicker starts and reduced possibility of engine starting damage.
However, a CVT transmission can contribute significantly more parasitic loss compared to the drive shaft during a start-up. Simply relieving the hydraulic pressure on the drive train, as with the powershift cold start bypass system, does not provide the magnitude of parasitic loss reduction needed to reliably allow the engine to efficiently start.
A centrifugal clutch was initially considered as a way to reduce CVT transmission parasitic loss demands on a cold start up. However, designing a centrifugal clutch to achieve the significant parasitic loss reduction needed at very low engine speeds at cold start up would also render it susceptible to constant slippage and wear at normal operating conditions, resulting in premature failure.
Various decoupling clutches have been used in transmissions to relieve cold start problems, such as one described in U.S. Pat. No. 5,174,109. This patent describes a coupling/decoupling clutch that provides for complete engagement and disengagement between a gas turbine power source and the loads driven by the power source. The disadvantage of applying complete disengagement to a CVT is that when engagement is desired, a relatively complicated control system is needed to engage the hydrostatic system as well as the mechanical system. In addition, at startup, the lag time for bringing those systems up to speed is lengthy. This lag time is a distinct disadvantage for vehicles operating in real time with a CVT.
A power system such as an engine for a vehicle has a limited amount of torque capability during a starting event. If the downstream powertrain components draw too much torque, the engine may not start or some of the engine components may suffer from premature wear. This situation is greatly exacerbated under cold ambient conditions. There is a need for a transmission system and method for the starting of engines with combined mechanical and hydrostatic transmissions such as with continuously variable transmissions (CVT) that provides a simple, economical, yet effective means of unloading the engine during a start. This disclosure addresses that need.